Method to align mask patterns

ABSTRACT

Alignment tolerances between narrow mask lines, for forming interconnects in the array region of an integrated circuit, and wider mask lines, for forming interconnects in the periphery of the integrated circuit, are increased. The narrow mask lines are formed by pitch multiplication and the wider mask lines are formed by photolithography. The wider mask lines and are aligned so that one side of those lines is flush with or inset from a corresponding side of the narrow lines. Being wider, the opposite sides of the wider mask lines protrude beyond the corresponding opposite sides of the narrow mask lines. The wider mask lines are formed in negative photoresist having a height less than the height of the narrow mask lines. Advantageously, the narrow mask lines can prevent expansion of the mask lines in one direction, thus increasing alignment tolerances in that direction. In the other direction, use of photolithography and a shadowing effect caused by the relative heights of the photoresist and the narrow mask lines causes the wider mask lines to be formed with a rounded corner, thus increasing alignment tolerances in that direction by increasing the distance to a neighboring narrow mask line.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.10/934,317, filed Sep. 2, 2004.

REFERENCE TO RELATED APPLICATIONS

This application is related to the following: U.S. patent applicationSer. No. 10/931,772 to Abatchev et al., filed Aug. 31, 2004; U.S. patentapplication Ser. No. 10/932,993 to Abatchev et al., filed Sep. 1, 2004;U.S. Pat. No. 7,115,525 to Abatchev et al., issued Oct. 3, 2006; andU.S. Pat. No. 7,151,040 to Tran et al., issued Dec. 19, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to integrated circuit fabrication and,more particularly, to masking techniques.

2. Description of the Related Art

As a consequence of many factors, including demand for increasedportability, computing power, memory capacity and energy efficiency inmodern electronics, integrated circuits are continuously being reducedin size. To facilitate this size reduction, the sizes of the constituentfeatures, such as electrical devices and interconnect line widths, thatform the integrated circuits are also constantly being decreased.

The trend of decreasing feature size is evident, for example, in memorycircuits or devices such as dynamic random access memories (DRAMs),static random access memories (SRAMs), ferroelectric (FE) memories, etc.To take one example, DRAM typically comprises millions of identicalcircuit elements, known as memory cells. In its most general form, amemory cell typically consists of two electrical devices: a storagecapacitor and an access field effect transistor. Each memory cell is anaddressable location that can store one bit (binary digit) of data. Abit can be written to a cell through the transistor and read by sensingcharge on the storage electrode from the reference electrode side. Bydecreasing the sizes of constituent electrical devices and theconducting lines that access them, the sizes of the memory devicesincorporating these features can be decreased. Additionally, storagecapacities can be increased by fitting more memory cells into the memorydevices.

The continual reduction in feature sizes places ever greater demands ontechniques used to form the features. For example, photolithography iscommonly used to pattern features, such as conductive lines, on asubstrate. The concept of pitch can be used to describe the size ofthese features. Pitch is defined as the distance between an identicalpoint in two neighboring features. These features are typically definedby spaces between adjacent features, which are typically filled by amaterial, such as an insulator. As a result, pitch can be viewed as thesum of the width of a feature and of the width of the space separatingthat feature from a neighboring feature. Due to factors such as opticsand light or radiation wavelength, however, photolithography techniqueseach have a minimum pitch below which a particular photolithographictechnique cannot reliably form features. Thus, the minimum pitch of aphotolithographic technique can limit feature size reduction.

“Pitch doubling” is one method proposed for extending the capabilitiesof photolithographic techniques beyond their minimum pitch. Such amethod is illustrated in FIGS. 1A-1F and described in U.S. Pat. No.5,328,810, issued to Lowrey et al., the entire disclosure of which isincorporated herein by reference. With reference to FIG. 1A,photolithography is first used to form a pattern of lines 10 in aphotoresist layer overlying a layer 20 of an expendable material and asubstrate 30. As shown in FIG. 1B, the pattern is then transferred by anetch step (preferably anisotropic) to the layer 20, formingplaceholders, or mandrels, 40. The photoresist lines 10 can be strippedand the mandrels 40 can be isotropically etched to increase the distancebetween neighboring mandrels 40, as shown in FIG. 1C. A layer 50 ofmaterial is subsequently deposited over the mandrels 40, as shown inFIG. 1D. Spacers 60, i.e., material extending or originally formedextending from sidewalls of another material, are then formed on thesides of the mandrels 40 by preferentially etching the spacer materialfrom the horizontal surfaces 70 and 80 in a directional spacer etch, asshown in FIG. 1E. The remaining mandrels 40 are then removed, leavingbehind the freestanding spacers 60, which together act as an etch maskfor patterning underlying layers, as shown in FIG. 1F. Thus, where agiven pitch formerly included a pattern defining one feature and onespace, the same width now includes two features and two spaces definedby the spacers 60. As a result, the smallest feature size possible witha photolithographic technique is effectively decreased.

It will be appreciated that while the pitch is actually halved in theexample above, this reduction in pitch is conventionally referred to aspitch “doubling,” or, more generally, pitch “multiplication.” That is,conventionally “multiplication” of pitch by a certain factor actuallyinvolves reducing the pitch by that factor. The conventional terminologyis retained herein.

The critical dimension of a mask scheme or circuit design is thescheme's minimum feature dimension. Due to factors such as geometriccomplexity and different requirements for critical dimensions indifferent parts of an integrated circuit, typically not all features ofthe integrated circuit will be pitch multiplied. Consequently, pitchmultiplied features will often need to be connected to or otherwisealigned with respect to non-pitch multiplied features in some part ofthe integrated circuit. Because these non-pitch multiplied featuresgenerally have larger critical dimensions than the pitch multipliedfeatures, the margin of error for aligning the non-pitch multipliedfeatures to contact the pitch multiplied features can be small.Moreover, because the critical dimensions of pitch-multiplied lines maybe near the resolution and/or overlay limits of many photolithographictechniques, shorting neighboring pitch multiplied features is anever-present possibility. Such shorts can undesirably cause theintegrated circuit to malfunction.

Accordingly, there is a need for methods which allow increased marginsof error for forming contacts between features of different sizes,especially for forming contacts between pitch multiplied and non-pitchmultiplied features.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method is provided forforming an integrated circuit. The method comprises providing asubstrate with an overlying temporary layer and a photodefinable layeroverlying the temporary layer. A pattern is formed in the photodefinablelayer and transferred to the temporary layer to form a plurality ofplaceholders in the temporary layer. A blanket layer of spacer materialis then deposited over the plurality of placeholders. The spacermaterial is selectively removed from horizontal surfaces. Theplaceholders are selectively removed relative to the spacer material toform a plurality of spacer loops. The spacer loops are etched to form apattern of separated spacers. An other photodefinable layer is formedaround and on the same level as the separated spacers. Interconnects arepatterned in the other photodefinable layer. The interconnects are widerthan the spacers and contact the spacers and are aligned with one longside of the interconnects inset from or collinear with a correspondinglong side of the spacers.

According to another aspect of the invention, a method is provided forsemiconductor fabrication. The method comprises providing a substrateand forming an elongated spacer over the substrate by pitchmultiplication. A photoresist line is then formed. The photoresist lineis in contact with an end of the spacer in a contact region. The spacerforms a boundary for only two faces of the photoresist line in thecontact region.

According to yet another aspect of the invention, a process is providedfor fabricating an integrated circuit. The process comprises forming afirst plurality of mask lines and forming a photodefinable layer aroundthe mask lines. The photodefinable layer has a thickness less than aheight of the mask lines. A plurality of features is patterned in thephotodefinable layer.

According to another aspect of the invention, a partially fabricatedintegrated circuit is provided. The partially fabricated integratedcircuit comprises a substrate and a first plurality of mask linesoverlying the substrate. A photodefinable layer contacts the mask lines.The photodefinable layer has a thickness less than a height of the masklines.

According to yet another aspect of the invention, an integrated circuitis provided. The integrated circuit comprises a plurality ofinterconnects, each having a first portion with a first width in anarray region and a second portion with a second width in a peripheryregion. The second width is larger than the first width. An end of eachof the second interconnect portions contacts an end of each of the firstinterconnect portions. One side of each of the second interconnectportions, extending along a length of one of the second interconnectportions, is substantially collinear with or inset from one side of eachof the first interconnect portions, extending along a length of one ofthe first interconnect portions.

According to another aspect of the invention, an integrated circuit isprovided. The integrated circuit comprises a plurality of interconnectseach having a first portion and a second portion. The first portion ofthe interconnects are substantially parallel to each other between firstand second spaced planes extending perpendicular to the lines. Inaddition, the first portion and the second portion contact in an overlapregion. Only one corner of the second portion protrudes beyond a side ofthe first portion in the overlap region.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the Detailed Description ofthe Preferred Embodiments and from the appended drawings, which aremeant to illustrate and not to limit the invention, and wherein:

FIGS. 1A-1F are schematic, cross-sectional side views of mask lines,formed in accordance with a prior art pitch multiplication method;

FIGS. 2A and 2B are schematic, top plan views of conductive lines in anintegrated circuit;

FIGS. 3A-3D are schematic, top plan views of a partially formedintegrated circuit, in accordance with preferred embodiments of theinvention;

FIG. 3E is a schematic, cross-sectional side view the partially formedintegrated circuit of FIGS. 3A, 3B, 3C or 3D, in accordance withpreferred embodiments of the invention;

FIGS. 4A-4B are schematic, top plan and cross-sectional side views,respectively, of a partially formed integrated circuit, in accordancewith preferred embodiments of the invention;

FIG. 5 is a schematic, cross-sectional side view of the partially formedmemory device of FIGS. 4A-4B after forming lines in a selectivelydefinable layer, in accordance with preferred embodiments of theinvention

FIG. 6 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 5 after widening spaces between photoresistlines, in accordance with preferred embodiments of the invention;

FIG. 7 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 6 after etching through a hard mask layer, inaccordance with preferred embodiments of the invention;

FIG. 8 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 7 after transferring a pattern from thephotoresist and hard mask layers to a temporary layer, in accordancewith preferred embodiments of the invention;

FIG. 9 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 8 after depositing a blanket layer of aspacer material, in accordance with preferred embodiments of theinvention;

FIG. 10 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 9 after a spacer etch, in accordancewith preferred embodiments of the invention;

FIG. 11 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 10 after removing remnants of thetemporary layer from between spacers, in accordance with preferredembodiments of the invention;

FIG. 12 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 11 after depositing a protectivelayer, in accordance with preferred embodiments of the invention;

FIG. 13 is a schematic, top plan view of the partially formed integratedcircuit of FIG. 12 after patterning the protective layer, in accordancewith preferred embodiments of the invention;

FIGS. 14A-14B are schematic, top plan and cross-sectional side views ofthe partially formed integrated circuit of FIG. 13 after etching exposedportions of spacers, in accordance with preferred embodiments of theinvention;

FIGS. 15A-15B are schematic, top plan and cross-sectional side views ofthe partially formed integrated circuit of FIG. 13 after removing theprotective layer, in accordance with preferred embodiments of theinvention;

FIG. 16 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIGS. 15A-15B after depositing aphotodefinable layer, in accordance with preferred embodiments of theinvention;

FIGS. 17A-17B are schematic, top plan and cross-sectional side views ofthe partially formed integrated circuit of FIG. 16 after patterning thephotodefinable layer, in accordance with preferred embodiments of theinvention;

FIG. 18 is a schematic, top plan view of the partially formed integratedcircuit of FIGS. 17A-17B showing overdevelopment of the photodefinablelayer, in accordance with some preferred embodiments of the invention;

FIG. 19 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIGS. 17A-17B after etching through a hardmask layer, in accordance with preferred embodiments of the invention;

FIG. 20 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 18 after etching the additionalmasking layer, in accordance with preferred embodiments of theinvention; and

FIG. 21 is a schematic, top plan view of the partially formed integratedcircuit of FIG. 20 after forming conductive interconnects in thesubstrate, in accordance with preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Typically, separately defined features contacting lines are nominallycentered perfectly on the lines to maximize the separation of thefeatures from neighboring pitch-multiplied lines. FIGS. 2A and 2B showssuch an idealized alignment of pitch multiplied lines 91 and non-pitchmultiplied lines 92. FIG. 2A shows an arrangement in which every line 91is contacted on the same side with a line 92 and FIG. 2B shows anarrangement in which lines 92 contact the lines 91 on alternating sides.

Various types of misalignments can occur in the overlap region 93 inwhich the lines 91 and 92 contact, however. For example, the lines 92may all be misaligned in one direction. Because the lines 92 are widerthan the space separating the lines 91, a single line 92 can short twolines 91 if the misalignment is severe. Another form of misalignment canoccur when a line 92 is skewed in one direction, relative to other lines92. This can also cause a short if neighboring lines 92 contact. Inreality, all these forms of misalignments can occur concurrently. Inaddition, because the lines 91 are also formed with a certain margin oferror in size and location, any misalignment of the lines 92 may beexacerbated by misalignments or deviations in size of the lines 91.Thus, to minimize the possibility of shorts or otherwise sub-optimalcontacts, the tolerances for misalignments of the lines 92 and 91 arepreferably large. For lines 92 and 94 each having a given width, thedistances 94 between each line 92 and each line 91 and the distances 95between neighboring lines 92 are preferably each maximized to maximizetolerances. More preferably, both of the distances 94 and 95 aremaximized. It will be appreciated that the same principles apply whenthe features are contacts, contact pads or other features that are to bekept separate.

Reference will now be made to the Figures, wherein like numerals referto like parts throughout. It will be appreciated that the Figures arenot necessarily drawn to scale.

With reference initially to FIGS. 3A and 3B, preferred embodiments ofthe invention advantageously allow for increased misalignmenttolerances. Preferably, a first feature, such as a line 91, is formed ina first mask layer and a second feature, such as a line 92, is overlaidthe first feature by patterning a second masking layer. Rather thancenter the second feature on the first feature, the second feature isaligned so that only one of its sides 92 a extends beyond acorresponding side 91 a of the first feature, as illustrated in FIG. 3A.The other side 92 b is shown inset from the corresponding side 91 b ofthe first feature. Thus, only part of the end of the smaller first line91 is overlapped with the end of the wider second line 92 and only onecorner of the line 92 protrudes beyond a side of the line 91. In otherpreferred embodiments, as illustrated in FIG. 3B, side 92 b preferablyis positioned substantially flush or collinear with side 91 b, whileside 92 a is positioned beyond side 91 a. It will be appreciated thatthe lines 92 can also contact the lines 91 on alternating sides, asshown in FIGS. 3C and 3D. Moreover, because the lines 92 alternate onany given side of the lines 91, the corners 96 can protrude beyondeither side 91 a or 91 b or some corners 96 can protrude beyond the side91 a and others beyond the side 91 b, since concerns of two neighboringcorners 96 shorting are minimized by the alternating arrangement.

Thus, preferably only one corner 96 of the wider second featureprotrudes substantially beyond the narrow first feature. As discussedbelow, the protruding corner 96 can be formed rounded. By utilizing thisrounding, the distance, e.g., distance 94, between neighboring features,e.g., the lines 91 and 92, in the overlap region, e.g., region 93, canbe increased relative to lines 92 without rounding.

Moreover, as also discussed below, the second feature is preferablyformed in photoresist by photolithography. The photoresist is preferablyformed below the vertical height of the first feature, as illustrated inFIG. 3E for lines 91 and 92, the lines 91 having a height 97. As such,the first features, such as the lines 91, act as walls that permit thesecond features, such as the lines 92, to grow, if at all, in only onedirection. Thus, by limiting the potential growth of the second featuresand by geometrically positioning the mask features to maximize thedistance between neighboring features, misalignment tolerances can beincreased.

Advantageously, any misalignments can be weighted in one direction,e.g., in the direction of the sides 91 b, while still maintaining anadequate distance 94 between lines 92 and 91. Moreover, the photoresistcan be overdeveloped to narrow the second features to further minimizethe possibility of shorting. It will be appreciated that theoverdevelopment can include any process which reduces the criticaldimension of the lines 92. For example, the overdevelopment can includeextending the development period and/or performing an additionaltreatment or technique to controllably remove resist, e.g., partiallyexposed resist. For example, the overdevelopment can remove resist inthe exposure threshold or “gray” area between the area fully exposed toradiation transmitted through the reticle and lens system and the areathat is “dark” or exposed to a light intensity below the exposurethreshold for the resist.

While not limited by theory, three effects have been found to beadvantageously present in the preferred embodiments. It will beappreciated that each of these effects are independently advantageousand are not each necessarily present in all embodiments. Nevertheless,all three effects can be found in particularly advantageous embodiments.

Preferably, the first mask features, such as the lines 91, are firstformed and then the second mask features, such as the lines 92, areoverlaid the first features. The second features are preferablypatterned in photoresist that overlaps the first features. It will beappreciated that the photoresist is typically patterned by being exposedto radiation through a reticle and then developed. In the case ofnegative photoresist, radiation, e.g., light, is focused on parts of thephotoresist which are to be retained, e.g., on the areas where the lines92 are to be formed. Typically, the radiation activates a photosensitivecompound, e.g., a photo-induced acid generator (PAG), which decreasesthe solubility of the photoresist, e.g., by causing it to polymerize.Such photoactivated chemicals, however, can diffuse, thereby causing thelines 92 to expand to areas that are not irradiated. In addition todecreasing the precision with which the lines 92 are formed, thisdiffusion can also cause the lines 92 to expand, thereby decreasingalignment tolerances by decreasing distances 94 between the lines 91 and92.

In the preferred embodiments, the photoresist is preferably formed at alevel below the top of the first mask features, such as the lines 91.The photoresist is then patterned to form the second features.Advantageously, in this arrangement, the first mask features act as awall, only allowing diffusion of photogenerated acid from PAG's in adirection away from those first mask features. Thus, the misalignmenttolerance in the region 93 can be increased by an amount approximatelyequal to the amount that the photogenerated acids would have otherwisediffused to expand the lines 92. Since diffusion can only occur in onedirection, the risk of shorting from diffusion is effectively halved inthe overlap region.

In addition, due to various factors, including diffusion ofphotogenerated acids and diffraction of radiation, photolithography willform corners that are rounded. Thus, by aligning features such as thelines 91 and 92 so that each narrow line 91 is closest to a roundedcorner of a neighboring line 92, the rounding effect can be used toincrease the distance between the wider lines 92 and adjacent narrowerlines 91. This advantageously also increases misalignment margins byincreasing corner-to-corner distances, such as the distance 94.

Also, while the preferred embodiments may be applied using anyphotodefinable material, including positive or negative photoresist,negative photoresist is utilized in particularly advantageousembodiments. Advantageously, by using negative photoresist to form thelines 92 and forming that photoresist below the level of the secondfeatures, such as the lines 91, the rounding effect can be increased. Itwill be appreciated that light reaching the photoresist between lines 91is attenuated due to a shadowing affect caused by the presence of thetaller lines 91. Moreover, this attenuation will increase withincreasing distance down the lines 91 away from the ends of the lines91. Advantageously, because the formation of the lines 92 depends uponlight hitting the negative photoresist, the attenuation further roundsoff and narrows the width of the lines 92 between lines 91. As a result,the distances between the wider lines 92 and the neighboring narrowerlines 91 can be further increased, thereby further increasingmisalignment margins.

More preferably, a combination of positive and negative photoresist isused, with the positive photoresist used in forming (preferably via aprocess utilizing spacers) the lines 91 and the negative photoresistusing in forming the lines 92. The negative photoresist advantageouslyincreases the rounding discussed above, while the positive photoresistadvantageously allows a higher resolution relative to the negativephotoresist.

FIG. 4A shows a top view of an integrated circuit 100, which ispreferably a memory chip. A central region 102, the “array,” issurrounded by a peripheral region 104, the “periphery.” It will beappreciated that, in a fully formed integrated circuit 100, the array102 will typically be densely populated with conducting lines andelectrical devices such as transistors and capacitors. In a memorydevice, the electrical devices form a plurality of memory cells, whichare typically arranged in a regular pattern, such as rows. Desirably,pitch multiplication can be used to form features such as rows/columnsof transistors, capacitors or interconnects in the array 102, asdiscussed below. On the other hand, the periphery 104 typicallycomprises features larger than those in the array 102. Conventionalphotolithography, rather than pitch multiplication, is preferably usedto pattern features, such as logic circuitry, in the periphery 104,because the geometric complexity of logic circuits located in theperiphery 104 makes using pitch multiplication difficult, whereas theregular grid typical of memory array patterns is conducive to pitchmultiplication. In addition, some devices in the periphery requirelarger geometries due to electrical constraints, thereby making pitchmultiplication less advantageous than conventional photolithography forsuch devices. It will be appreciated that the periphery 104 and thearray 102 are not draw to scale and their relative positions may varyfrom that depicted.

With reference to FIG. 4B, a partially formed integrated circuit 100 isprovided. A substrate 110 is provided below various masking layers120-160. The layers 120-160 will be etched to form a mask for patterningthe substrate 110 to form various features, as discussed below.

The materials for the layers 120-160 overlying the substrate 110 arepreferably chosen based upon consideration of the chemistry and processconditions for the various pattern forming and pattern transferringsteps discussed herein. Because the layers between a topmost selectivelydefinable layer 120, which preferably is definable by a lithographicprocess, and the substrate 110 will preferably function to transfer apattern derived from the selectively definable layer 120 to thesubstrate 110, the layers between the selectively definable layer 120and the substrate 110 are preferably chosen so that they can beselectively etched relative to other exposed materials. It will beappreciated that a material is considered selectively, orpreferentially, etched when the etch rate for that material is at leastabout 5 times greater, preferably about 10 times greater and, mostpreferably, at least about 40 times greater than that for surroundingmaterials.

In the illustrated embodiment, the selectively definable layer 120overlies a first hard mask, or etch stop, layer 130, which overlies atemporary layer 140, which overlies a second hard mask, or etch stop,layer 150, which overlies an additional mask layer 160, which overliesthe substrate 110 to be processed (e.g., etched) through a mask.Preferably, the mask through which the substrate 110 is processed isformed in the second hard mask layer 150 or in the additional mask layer160.

It will be understood that in common methods of transferring patterns,both the mask and the underlying substrate are exposed to an etchant,which preferentially etches away the substrate material. The etchants,however, also wear away the mask materials, albeit at a slower rate.Thus, over the course of transferring a pattern, the mask can be wornaway by the etchant before the pattern transfer is complete. Thesedifficulties are exacerbated where the substrate 110 comprises multipledifferent materials to be etched. In such cases, the additional masklayer 160, which preferably comprises a thick layer of amorphous carbon,is desirable to prevent the mask pattern from being worn away before thepattern transfer is complete. The illustrated embodiment shows the useof the additional mask layer 160.

It will also be understood, however, that because the various layers arechosen based upon the requirements of chemistry and process conditions,one or more of the layers can be omitted in some embodiments. Forexample, the additional mask layer 160 can be omitted in embodimentswhere the substrate 110 is relatively simple, e.g., where the substrate110 is a single layer of material and where the depth of the etch ismoderate. In such cases, the second hard mask layer 150 may be asufficient mask for transferring a pattern to the substrate 110.Similarly, for a particularly simple substrate 110, the various otherlayers, such the second hard mask layer 150 itself, may be omitted andoverlying mask layers may be sufficient for the desired patterntransfer. The illustrated sequence of layers, however, is particularlyadvantageous for transferring patterns to difficult to etch substrates,such as a substrate 110 comprising multiple materials or multiple layersof materials, or for forming small and high aspect ratio features.

With reference to FIG. 2, the selectively definable layer 120 ispreferably formed of a photoresist, including any photoresist known inthe art. For example, the photoresist can be any photoresist compatiblewith 13.7 nm, 157 nm, 193 nm, 248 nm or 365 nm wavelength systems, 193nm wavelength immersion systems or electron beam lithographic systems.Examples of preferred photoresist materials include argon fluoride (ArF)sensitive photoresist, i.e., photoresist suitable for use with an ArFlight source, and krypton fluoride (KrF) sensitive photoresist, i.e.,photoresist suitable for use with a KrF light source. ArF photoresistsare preferably used with photolithography systems utilizing relativelyshort wavelength light, e.g., 193 nm. KrF photoresists are preferablyused with longer wavelength photolithography systems, such as 248 nmsystems. In other embodiments, the layer 120 and any subsequent resistlayers can be formed of a resist that can be patterned by nano-imprintlithography, e.g., by using a mold or mechanical force to pattern theresist.

The material for the first hard mask layer 130 preferably comprises aninorganic material, and exemplary materials include silicon oxide(SiO₂), silicon or a dielectric anti-reflective coating (DARC), such asa silicon-rich silicon oxynitride. In the illustrated embodiment, thefirst hard mask layer 130 is a dielectric anti-reflective coating(DARC). The temporary layer 140 is preferably formed of amorphouscarbon, which offers very high etch selectivity relative to thepreferred hard mask materials. More preferably, the amorphous carbon isa form of transparent carbon that is highly transparent to light andwhich offers further improvements for photo alignment by beingtransparent to wavelengths of light used for such alignment. Depositiontechniques for forming a highly transparent carbon can be found in A.Helmbold, D. Meissner, Thin Solid Films, 283 (1996) 196-203, the entiredisclosure of which is incorporated herein by reference.

Advantageously, using DARCs for the first hard mask layer 130 can beparticularly advantageous for forming patterns having pitches near theresolution limits of a photolithographic technique. The DARCs canenhance resolution by minimizing light reflections, thus increasing theprecision with which photolithography can define the edges of a pattern.Optionally, a bottom anti-reflective coating (BARC) (not shown) cansimilarly be used in addition to the first hard mask layer 130 tocontrol light reflections.

The second hard mask layer 150 preferably comprises a dielectricanti-reflective coating (DARC) (e.g., a silicon oxynitride), silicon oraluminum oxide (Al₂O₃). A bottom anti-reflective coating (BARC) (notshown) can optionally be used to control light reflections. In addition,like the temporary layer 140, the additional mask layer 160 ispreferably formed of amorphous carbon due to its excellent etchselectivity relative to many materials.

In addition to selecting appropriate materials for the various layers,the thicknesses of the layers 120-160 are preferably chosen dependingupon compatibility with the etch chemistries and process conditionsdescribed herein. For example, when transferring a pattern from anoverlying layer to an underlying layer by selectively etching theunderlying layer, materials from both layers are removed to some degree.Thus, the upper layer is preferably thick enough so that it is not wornaway over the course of the pattern transfer.

In the illustrated embodiment, the selectively definable layer 120 is aphotodefinable layer preferably between about 50-300 nm thick and, morepreferably, between about 200-250 nm thick. The first hard mask layer130 is preferably between about 100-400 nm thick and, more preferably,between about 150-300 nm thick. The temporary layer 140 is preferablybetween about 100-200 nm thick and, more preferably, between about120-150 nm thick. The second hard mask layer 150 is preferably betweenabout 20-80 nm thick and, more preferably, about 50 nm thick and theadditional mask layer 160 is preferably between about 200-500 nm thickand, more preferably, about 300 nm thick.

The various layers discussed herein can be formed by various methodsknown to those of skill in the art. For example, various vapordeposition processes, such as chemical vapor deposition, can be used toform hard mask layers. Preferably, a low temperature chemical vapordeposition process is used to deposit the hard mask layers or any othermaterials, e.g., spacer material, over the mask layer 160, where themask layer 160 is formed of amorphous silicon. Such low temperaturedeposition processes advantageously prevent chemical or physicaldisruption of the amorphous carbon layer. Spin-on-coating processes canbe used to form photodefinable layers. In addition, amorphous carbonlayers can be formed by chemical vapor deposition using a hydrocarboncompound, or mixtures of such compounds, as carbon precursors. Exemplaryprecursors include propylene, propyne, propane, butane, butylene,butadiene and acetelyne. A suitable method for forming amorphous carbonlayers is described in U.S. Pat. No. 6,573,030 B1, issued to Fairbairnet al. on Jun. 3, 2003, the entire disclosure of which is incorporatedherein by reference. In addition, the amorphous carbon may be doped. Asuitable method for forming doped amorphous carbon is described in U.S.patent application Ser. No. 10/652,174 to Yin et al., the entiredisclosure of which is incorporated herein by reference.

In a first phase of methods in accordance with the preferred embodimentsand with reference to FIGS. 4-11, a pattern of spacers is formed bypitch multiplication.

With reference to FIG. 5, a pattern comprising spaces or trenches 122delimited by photodefinable material features 124 is formed in thephotodefinable layer 120. The trenches 122 can be formed by, e.g.,photolithography, in which the layer 120 is exposed to radiation througha reticle and then developed. After being developed, the remainingphotodefinable material, photoresist in the illustrated embodiment,forms mask features such as the illustrated lines 124 (shown incross-section only).

The pitch of the resulting lines 124 is equal to the sum of the width ofa line 124 and the width of a neighboring space 122. To minimize thecritical dimensions of features formed using this pattern of lines 124and spaces 122, the pitch is preferably at or near the limits of thephotolithographic technique used to pattern the photodefinable layer120. For example, for photolithography utilizing 248 nm light, the pitchof the lines 124 can be about 100 nm. Thus, the pitch may be at theminimum pitch of the photolithographic technique and the spacer patterndiscussed below can advantageously have a pitch below the minimum pitchof the photolithographic technique.

As shown in FIG. 6, the spaces 122 can optionally be widened or narrowedto a desired dimension. For example, the spacers 122 can be widened byetching the photoresist lines 124, to form modified spaces 122 a andlines 124 a. The photoresist lines 124 are preferably etched using anisotropic etch, such as a sulfur oxide plasma, e.g., a plasma comprisingSO₂, O₂, N₂ and Ar. The extent of the etch is preferably selected sothat the widths of the lines 124 a are substantially equal to thedesired spacing between the later-formed spacers 175, as will beappreciated from the discussion of FIGS. 9-11 below. Advantageously,this etch allows the lines 124 a to be narrower than would otherwise bepossible using the photolithographic technique used to pattern thephotodefinable layer 120. In addition, the etch can smooth the edges ofthe lines 124 a, thus improving the uniformity of those lines. In otherembodiments, the spaces between the spaces 122 can be narrowed byexpanding the lines 124 to a desired size. For example, additionalmaterial can be deposited over the lines 124 or the lines 124 can bechemically reacted to form a material having a larger volume to increasetheir size.

The pattern in the (modified) photodefinable layer 120 is preferablytransferred to the temporary layer 140 to allow for deposition of alayer 170 of spacer material (FIG. 9). The temporary layer 140 ispreferably formed of a material that can withstand the processconditions for spacer material deposition and etch, discussed below. Inother embodiments where the deposition of spacer material is compatiblewith the photodefinable layer 120, the temporary layer 140 can beomitted and the spacer material can be deposited directly on thephoto-defined features 124 or the modified photodefined features 124 aof the photodefinable layer 120 itself.

In the illustrated embodiment, in addition to having higher heatresistance than photoresist, the material forming the temporary layer140 is preferably selected such that it can be selectively removedrelative to the material for the spacers 175 (FIG. 10) and theunderlying etch stop layer 150. As noted above, the layer 140 ispreferably formed of amorphous carbon.

The pattern in the photodefinable layer 120 is preferably firsttransferred to the hard mask layer 130, as shown in FIG. 7. Thistransfer is preferably accomplished using an anisotropic etch, such asan etch using a fluorocarbon plasma, although a wet (isotropic) etch mayalso be suitable if the hard mask layer 130 is thin. Preferredfluorocarbon plasma etch chemistries include CF₄, CFH₃, CF₂H₂ and CF₃H.

The pattern in the photodefinable layer 120 is then transferred to thetemporary layer 140, as shown in FIG. 8, preferably using aSO₂-containing plasma, e.g., a plasma containing SO₂, O₂ and Ar.Advantageously, the SO₂-containing plasma can etch carbon of thepreferred temporary layer 140 at a rate greater than 20 times and, morepreferably, greater than 40 times the rate that the hard mask layer 130is etched. A suitable SO₂-containing plasma is described in U.S. patentapplication Ser. No. 10/931,772 to Abatchev et al., filed Aug. 31, 2004,entitled Critical Dimension Control, the entire disclosure of which isincorporate herein by reference. It will be appreciated that theSO₂-containing plasma can simultaneously etch the temporary layer 140and also remove the photodefinable layer 120. The resulting lines 124 bconstitute the placeholders or mandrels along which a pattern of spacers175 (FIG. 10) will be formed.

Next, as shown in FIG. 9, a layer 170 of spacer material is preferablyblanket deposited conformally over exposed surfaces, including the hardmask layer 130, the hard mask 150 and the sidewalls of the temporarylayer 140. Optionally, the hard mask layer 130 can be removed beforedepositing the layer 170. The spacer material can be any material thatcan act as a mask for transferring a pattern to the underlying substrate110, or that otherwise can allow processing of underlying structuresthrough the mask being formed. The spacer material preferably: 1) can bedeposited with good step coverage; 2) can be deposited at a temperaturecompatible with the temporary layer 140; 3) can be selectively etchedrelative to the temporary layer 140 and any layer underlying thetemporary layer 140. Preferred materials include silicon oxides andnitrides. The spacer material is preferably deposited by chemical vapordeposition or atomic layer deposition. The layer 170 is preferablydeposited to a thickness of between about 20-60 nm and, more preferably,about 20-50 nm. Preferably, the step coverage is about 80% or greaterand, more preferably, about 90% or greater.

As shown in FIG. 10, the spacer layer 170 is then subjected to ananisotropic etch to remove spacer material from horizontal surfaces 180of the partially formed integrated circuit 100. Such an etch, also knownas a spacer etch, can be performed using HBr/Cl plasma. The etchincludes a physical component and preferably can also include a chemicalcomponent and can be, e.g., a reactive ion etch (RIE), such as a Cl₂,HBr etch. Such an etch can be performed, for example, using a LAMTCP9400 flowing about 0-50 sccm Cl₂ and about 0-200 sccm HBr at about7-60 mTorr pressure with about 300-1000 W top power and about 50-250 Wbottom power.

With reference to FIG. 11, the hard mask layer 130 (if still present)and the temporary layer 140 are next removed to leave freestandingspacers 175. The temporary layer 140 is selectively removed, preferablyusing a sulfur-containing plasma etch such as an etch using SO₂.

Thus, pitch multiplication has been accomplished. In the illustratedembodiment, the pitch of the spacers 175 is roughly half that of thephotoresist lines 124 and spaces 122 (FIG. 5) originally formed byphotolithography. Advantageously, spacers 175 having a pitch of about100 nm or less can be formed. It will be appreciated that because thespacers 175 are formed on the sidewalls of the features or lines 124 b,the spacers 175 generally follow the outline of the pattern of featuresor lines 124 a in the photodefinable layer 120 and, so, typically form aclosed loop.

Next, in a second phase of methods according to the preferredembodiments, the loops formed by the spacers 175 are separated intoindividual lines. This separation is preferably accomplished by an etchthat for each loop forms two separate lines of spacers 175 correspondingto two separate conductive paths in the substrate 110. It will beappreciated that more than two lines can be formed, if desired, byetching the spacers 175 at more than two locations.

To form the separate lines, a protective mask is preferably formed overparts of the lines to be retained and the exposed, unprotected parts ofthe spacer loops are then etched. The protective mask is then removed toleave a plurality of electrically separated lines.

With reference to FIG. 12, a protective material forming a secondprotective layer 300 is preferably deposited around and over the spacers175 and the parts of the layers 150 and 160. The protective material ispreferably a photodefinable material such as photoresist. Optionally, ananti-reflective coating (not shown) can be provided under the layer 300,e.g., directly above the substrate 110, to improve photolithographyresults. The photoresist and the anti-reflective coating can bedeposited using various methods known in the art, includingspin-on-coating processes.

With reference to FIG. 13, a protective mask 310 is subsequentlypatterned in the second protective layer 300, e.g., by photolithography,to protect desired parts of the underlying spacers 175 from a subsequentetch. To separate the spacers 175 of one loop into two separate lines,portions of the loops are exposed for etching in at least two separatelocations. To simplify processing, the exposed portions of the loops arepreferably the ends of the loops formed by the spacers 175, asillustrated.

In other embodiments, it will be appreciated that the protective layer300 can be formed of any material that can be selectively removed, e.g.,relative to the spacers 175, the layers 150-160 and the substrate 110.In those cases, the protective mask 310 can be formed in anothermaterial, e.g., photoresist, overlying the layer 300.

With reference to FIG. 14A, the exposed portions of the spacers 175 areetched away. Where the spacers 175 comprise silicon oxide or nitride,preferred etch chemistries include a fluorocarbon etch. After beingetched, the spacers 175 no longer form a loop with a neighboring spacer175. FIG. 14B shows a side view of the resulting structure, taken alongthe vertical plane 14B of FIG. 14A.

With reference to FIGS. 15A and 15B, the protective material ispreferably selectively removed. For example, the partially formedintegrated circuit 100 can be subjected to an ash process to remove theprotective material. It will be appreciated that the spacers 175 are notattacked during this removal step and that the layer 160 is protected bythe second hard mask layer 150. Thus, a plurality of individual masklines 320 are formed. FIG. 15A shows a top plan view of the resultantstructure and FIG. 15B shows a cross-sectional side view taken along thevertical plane 15B of FIG. 15A.

Next, in a third phase of methods according to the preferredembodiments, a second pattern is stitched or overlapped with the patternof spacers 320. In the illustrated embodiments, the second pattern isoverlaid on the spacers 320 to pattern features that will contact thefeatures defined by the spacers 320. To form this second pattern, aphotodefinable layer 400 is preferably formed around the spacers 320, asshown in FIG. 16. The photodefinable layer 400 preferably has a heightbelow the level of the spacers 320. Preferably, the photodefinable layer400 is formed of photoresist and, more preferably, negative photoresist.

The photoresist can be deposited to the desired height using a scancoating method, such as that commercially available from Tokyo ElectronLimited (TEL) of Tokyo, Japan. In other embodiments, a relatively thicklayer of photoresist can be deposited and then etched back to thedesired height. Preferably, the etch back does not activate PAG's orotherwise detrimentally effect the photoresist for subsequent exposureand development steps. The photodefinable layer 400 is preferably about80% or less and, more preferably, about 75-50% of the height of thespacers 320.

A pattern corresponding to line extensions or contacts to features to bedefined by the spacers 320 is next formed in the photodefinable layer400. It will be appreciated that the photodefinable layer 400 can bepatterned using any photolithographic technique, including the samephotolithographic technique used to pattern the photodefinable layer120. Preferably, the photodefinable layer 400 is patterned using ahigher resolution technique than the photodefinable layer 120. Forexample, where the photodefinable layer 120 is patterned using 248 nmphotolithography to form features having a pitch of about 100 nm, thephotodefinable layer 400 can be patterned using 193 nm photolithographyto form features having a pitch of about 140 nm. Preferably, thephotodefinable layer 400 is patterned using a lithographic techniquethat allows patterning with a pitch less than that of the spacers 320.For example, electron beam lithography, which has high resolution butrelatively poor alignment capabilities, can be used to pattern thephotodefinable layer 400.

With reference to FIGS. 17A and 17B, a pattern of features 410 is formedin the photodefinable layer 400. It will be appreciated that thefeatures 410 can be used to pattern features of different sizes than thespacers 320, including landing pads, local interconnects, etc. Asillustrated, the features 410 preferably overlap the pattern of spacers320 to ultimately form interconnects that contact interconnects formedusing the spacers 320. In the illustrate embodiment, the interconnectsformed using the features 410 have a larger critical dimension thanthose formed using the spacers 320.

FIG. 17A shows a top plan view of the resulting structure, with features410 and spacers 320. It will be appreciated that the shape of thefeatures 410 correspond to the location of light reaching thephotoresist layer 400 (FIG. 16), where the photoresist layer 400comprises negative photoresist. Where the photoresist layer 400comprises positive photoresist, light is directed to all areas of thepartially fabricated integrated circuit 100, except for the location ofthe features 410. Due to the corner rounding phenomena discussed above,the shape of the features 410 in the reticle (not shown) can berectangular, with sharp corners, while still advantageously formingrounded corners. More preferably, the reticle is shaped to ensurerounding of the corners, especially where positive photoresist is used.

In addition, the reticle can be positioned so that the features 410align with one of their long sides 410 a inset from the correspondinglong sides 320 a of the spacers 320 to ensure that the sides 410 a donot extend beyond the sides 320 a. Thus, while illustrated inset fromthe sides 320 a a certain distance, in other embodiments, due to thelimitations of photolithography, the sides 410 a may be further insetfrom the sides 320 a or may be collinear with the sides 320 a.Preferably, however, a targeted or desired position of the sides 410 ais collinear with or slightly inset from the sides 320 a.

As known in the art, the photoresist is developed after beingirradiated, leaving the pattern of features 410 shown. FIG. 17B shows across-sectional side view of the partially fabricated integrated circuit100, taken along the vertical plane 17B of FIG. 17A.

With reference to FIG. 18, in some embodiments the features 410 may belarger than desired (solid line) or may be positioned beyond the sides410 a. In such cases, the photoresist layer 400 can be overdeveloped tofurther narrow those features to a desired size (dotted line) and/orposition.

The pattern of features 410 and spacers 320 can next be transferred tothe additional mask layer 160. Preferably, the additional mask layer 160comprises a material having good etch selectivity to the material(s) ofthe substrate 110, and vice versa, to allow for an effective transferand later mask removal. To transfer to the pattern, the hard mask layer150 overlying the additional mask layer 160 is first etched (FIG. 19).The hard mask layer 150 is preferably anisotropically etched, preferablyusing a fluorocarbon plasma. Alternatively, an isotropic etch may beused if the hard mask layer 150 is relatively thin. While not shown, itwill be understood that in the array region 102, the hard mask 150remains only directly under the spacers 320. The additional mask layer160 is then anisotropically etched, preferably using a SO₂-containingplasma, which can simultaneously remove the features 410. FIG. 20 showsa schematic side view of the resulting partially formed integratedcircuit 100 in the region of overlap between the two patterns. Thepattern in the additional mask layer 160 can then be transferred to thesubstrate 110. Given the disparate materials typically used for theadditional mask layer 160 and the substrate 110 (e.g., amorphous carbonand silicon or silicon compounds, respectively), the pattern transfercan be readily accomplished using conventional etches appropriate forthe material or materials of the substrate 110.

Preferably, the substrate 110 comprises a conductor suitable for actingas electrical interconnects between electrical devices. For example, thesubstrate comprises doped silicon or a metal layer, such as an aluminumor copper layer. Thus, the mask lines 320 and 410 can directlycorrespond to the desired-placement of interconnects in the substrate110. FIG. 21 shows a resulting structure, with interconnects 420 formedin the partially formed integrated circuit 100. In other embodiments,the substrate can be an insulator and the location of mask features cancorrespond to the desired location of insulators in an integratedcircuit.

It will be appreciated that formation of contacts according to thepreferred embodiments offers numerous advantages. For example, becausethe spacers 320 block photogenerated chemicals, such as acid from PAG's,from diffusing in one direction, the features 410 are prevented fromgrowing in that direction in the region of contact with the spacers 320.Thus, the misalignment tolerance, especially in the direction in whichdiffusion is blocked, may be thought of as increasing by the amount thatthe features 410 would have otherwise expanded. In the other direction,the rounding of the corners, due partially to shadowing from the tallerspacers extending above the second resist layer, also increases themisalignment tolerance by increasing the distance between the spacers320 and the features 410. Moreover, given that diffusion is prevented inone direction, any misalignment can be weighted in that direction, sincefurther expansion in that direction is minimized, especially where thephotoresist is overdeveloped. Because the features 410 preferablycontact the spacers 320 both at butting ends and from the side, arelatively high contact area can be achieved. Preferably, asillustrated, the spacers 320 form a boundary for exactly two faces ofthe features 410, e.g., line end faces and on one side of the features320. Moreover, a high contact area can advantageously be maintained evenafter trimming by overdevelopment. For example, if the features 410 werecentered on the spacers 320, trimming may leave the features 410 onlycontacting the spacers 320 ends. The off-set positioning above, however,allows the features 410 to contact the spacers 320 both at butting endsand from the side, thereby increasing the contact area between those twoparts.

It will be appreciated that, in any of the steps described herein,transferring a pattern from a first level to a second level involvesforming features in the second level that generally correspond tofeatures on the first level. For example, the path of lines in thesecond level will generally follow the path of lines on the first leveland the location of other features on the second level will correspondto the location of similar features on the first level. The preciseshapes and sizes of features can vary from the first level to the secondlevel, however. For example, depending upon etch chemistries andconditions, the sizes of and relative spacings between the featuresforming the transferred pattern can be enlarged or diminished relativeto the pattern on the first level, while still resembling the sameinitial “pattern.” Thus, the transferred pattern is still considered tobe the same pattern as the initial pattern. In contrast, forming spacersaround mask features can change the pattern.

It will also be appreciated that various modifications of theillustrated embodiments are possible. For example, the pitch of thespacers 320 can be more than doubled. For example, further pitchmultiplication can be accomplished by forming additional spacers aroundthe spacers 320, then removing the spacers 320, then forming spacersaround the spacers that were formerly around the spacers the 175, and soon. An exemplary method for further pitch multiplication is discussed inU.S. Pat. No. 5,328,810 to Lowrey et al.

In addition, while illustrated equally spaced for ease of illustration,the spacers 320 or the features 410 can each be spaced at varyingdistances to each other. Moreover, while all are illustrated contactingthe spacers 320 from the same side, some of the features 410 can bepositioned on different, e.g., sides. Moreover, while illustratedparallel over their entire lengths for illustrate, the distance betweenthe spacers 320 can vary over some or all of their lengths. Preferably,however, the spacers 320, and resulting interconnects 420, are parallelover some distance, e.g., between first and second planes 500 and 502extending perpendicular to the spacers 320 (FIGS. 18 and 21). Forexample, the illustrated spacers 320 are preferably parallel in thearray region 102 of a DRAM to connect the tightly packed memory cells inthat array region. In addition, various parts of the spacers 320 can beformed at angles relative to other parts of the spacers 320. Similarly,parts of the features 410 can be formed at angles relative to thespacers 320 or to other parts of the features 410. For example, thefeatures 410 can diverge away from the points of contact with thespacers 320 to allow for increased separation between those features.

Also, more than two patterns, e.g., corresponding to landing pads andinterconnects, can be consolidated on a mask layer before transferringthe consolidated pattern to the substrate. In some embodiments,additional mask layers can be deposited above the layer 150. Forexample, patterns corresponding to landing pads and additionalinterconnects can be transferred to a supplemental mask layer (notshown) overlying the hard mask layer 150 and then an overlyingphotodefinable layer (not shown) can be patterned and the patterntransferred to underlying layers. The substrate 110 can then beprocessed through the resulting mask pattern.

It will be appreciated that while “processing” through the various masklayers preferably involve etching an underlying layer, processingthrough the mask layers can involve subjecting layers underlying themask layers to any semiconductor fabrication process. For example,processing can involve ion implantation, diffusion doping, depositing,or wet etching, etc. through the mask layers and onto underlying layers.In addition, the mask layers can be used as a stop or barrier forchemical mechanical polishing (CMP) or CMP can be performed on the masklayers to allow for both planarizing of the mask layers and etching ofthe underlying layers.

It will also be appreciated that while the preferred embodiments willfind application in any context in which features in different maskpatterns are overlaid one other, in particularly advantageousembodiments, features formed by pitch multiplication or employingspacers on soft or hard masks are “stitched” or aligned with featuresformed by a masking technique with a lower resolution. Preferably, pitchmultiplied mask features are made to contact features formed byconventional photolithography. Thus, the pitch multiplied featurespreferably have a pitch below the minimum pitch of the photolithographictechnique used for patterning the other features. In addition, while thepreferred embodiments can be used to form any integrated circuit, theyare particularly advantageously applied to form devices having arrays ofelectrical devices, including logic or gate arrays and volatile andnon-volatile memory devices such as DRAM, ROM or flash memory.

Moreover, the principles and advantages discussed herein are applicableto a variety of contexts in which two or more adjacent mask patterns areto be mated within overlapping regions.

Accordingly, it will be appreciated by those skilled in the art thatvarious other omissions, additions and modifications may be made to themethods and structures described above without departing from the scopeof the invention. All such modifications and changes are intended tofall within the scope of the invention, as defined by the appendedclaims.

1. A method for integrated circuit fabrication, comprising: providing aplurality of mandrels over a substrate; depositing a blanket layer ofspacer material over the mandrels; anisotropically etching the blanketlayer to form spacers on sidewalls of the mandrels; selectively removingthe mandrels to form a plurality of free-standing spacers; depositingnegative photoresist on a same level as the free-standing spacers, thenegative photoresist contacting the spacers; patterning the negativephotoresist; transferring a pattern defined by the free-standing spacersand the patterned negative photoresist to an underlying hard mask layer;and transferring the pattern to the substrate.
 2. The method of claim 1,wherein depositing negative photoresist leaves a layer of negativephotoresist having a top surface below a topmost part of the spacers. 3.The method of claim 1, wherein patterning the negative photoresistdefines mask lines, wherein each of the mask lines are wider than eachof the spacers, wherein each of the lines contact each of the spacersand are aligned with one long side inset from or collinear with acorresponding long side of the spacer contacted by the line.
 4. Themethod of claim 3, wherein the long sides of the lines are substantiallyco-linear with the long sides of the spacers.
 5. The method of claim 1,further comprising transferring the pattern to a primary hard mask layerafter transferring the pattern to the underlying hard mask layer andbefore transferring the pattern to the substrate.
 6. The method of claim1, wherein the mandrels are formed of a carbon-containing material. 7.The method of claim 6, wherein the mandrels are formed of amorphouscarbon.
 8. The method of claim 6, wherein depositing the blanket layercomprises depositing the spacer material by atomic layer deposition. 9.The method of claim 1, wherein the underlying hard mask layer comprisesamorphous carbon.
 10. A method for fabricating an integrated circuit,comprising: forming a plurality of mask features in an array region of apartially fabricated integrated circuit; depositing a negativephotoresist layer on a same level as the mask features; patterning thenegative photoresist to define additional mask features in a peripheryregion of the partially fabricated integrated circuit; transferring apattern defined by the mask features and the additional mask features toan underlying substrate.
 11. The method of claim 10, wherein thenegative photoresist layer has a thickness less than a height of themask features.
 12. The method of claim 11, wherein the thickness of thephotodefinable layer is about 80% or less than a height of the maskfeatures.
 13. The method of claim 12, wherein the thickness of thephotodefinable layer is about 75-50% of the height of the mask features.14. The method of claim 10, further comprising etching back thedeposited photoresist after depositing the negative photoresist layerand before patterning the negative photoresist.
 15. The method of claim10, wherein the mask features extend in spaced, generally parallelrelation to one another.
 16. The method of claim 10, wherein forming theplurality of mask features comprises performing pitch multiplication.17. A method for integrated circuit fabrication, comprising: providing aplurality of mask features in an array region of a partially fabricatedintegrated circuit; depositing a negative photoresist layer on a samelevel as the mask features; patterning the negative photoresist todefine additional mask features in a periphery region of the partiallyfabricated integrated circuit, the additional mask features having alarger critical dimension than the mask features; transferring a patterndefined by the mask features and the additional mask features to anunderlying substrate.
 18. The method of claim 17, wherein transferringthe pattern to the substrate defines conductive interconnects, the maskfeatures defining a plurality of first conductive interconnects, whereinthe additional mask features define a plurality of second conductiveinterconnects.
 19. The method of claim 18, wherein an end of each of thefirst conductive interconnects abuts an end of each of the secondconductive interconnects.
 20. The method of claim 19, wherein the endsof the second conductive interconnects contact an entire width of theends of the first conductive interconnects.
 21. The method of claim 18,wherein the ends of the second conductive interconnects contact lessthan an entire width of the ends of the first conductive interconnects.22. The method of claim 20, wherein the ends of the second conductiveinterconnects partially overlap the ends of the first conductiveinterconnect.
 23. A method for integrated circuit fabrication,comprising: providing a plurality of mask features over a substrate;providing a negative photoresist layer on a same level as the maskfeatures; patterning the negative photoresist to define additional maskfeatures by exposing selected regions of the negative photoresist tolight, the additional mask features having a larger critical dimensionthan the mask features, wherein patterning the negative photoresistrounds only a single corner of each additional mask feature in aninterface region between the mask features and the additional maskfeatures.
 24. The method of claim 23, wherein the mask features aredisposed in an array region of a partially fabricated integrated circuitand the additional mask features are disposed in a periphery region ofthe partially fabricated integrated circuit.
 25. The method of claim 23,wherein a height of the mask features is selected to cast a shadow onthe negative photoresist during patterning the negative photoresist,thereby causing rounding of the single corner of each mask feature.